The long range goal of this research is to gain more understanding of the extensive structural rearrangements and alterations in macromolecular synthesis that occur during mitosis in cultured human cells. With this in mind, we propose to investigate specifically aspects of nuclear reformation and cytoplasmic reutilization of mRNA in the metaphase to G1 transition. For these studies, a cultured human cell line, HeLa S3, is being used to explore this period in the cell cycle because large numbers of cells homogeneous for metaphase, which by incubation enter G1, are obtained without the immediate use of inhibitors. In particular nuclear reformation will be studied by the use of phospholipid precursors to probe nuclear envelope reformation and by use of a defined nuclear subfraction, the nuclear matrix, of probe reformation of other nuclear structures. With regard to phospholipid metabolism, lipid precursors are incorporated as mitotic cells complete division even in the absence of renewed protein synthesis, and this incorporation can be visualized by electron microscope autoradiography at least by early telophase. We shall continue to select candidate polypeptides that coisolate with the nuclear matrix fraction in order to determine their topographical relationships and fate during late mitosis. Our recent information indicates that the disaggregation of polyribosomes during mitosis results in the release of free mRNP particles free of ribosomes. Only a portion of the total free metaphase mRNP particles reappear in progeny G1 cell polyribosomes. This now permits additional analyses to determine whether or not there is a structural basis for reutilization of mRNP particles in G1 and conceivably, whether or not there is selective reutilization of mRNA. It is not clear which events are either obligatory for or merely temporally coincident with cell division. This type of information should not only help to sort this out but, more important, conceivably could shed light on regulatory events involved in or associated with mitosis.